The present invention relates to a catalyst for cleaning up the exhaust gases of a diesel engine. With the catalyst according to the invention the nitrogen oxides in the oxygen-rich exhaust gas of a diesel engine can be converted with optimal utilization of the reductive constituents contained in the exhaust gas.
Diesel engines emit exhaust gases that contain, besides unconsumed oxygen and harmless combustion products such as water and carbon dioxide, additional substances that are harmful and which endanger human health and pollute the environment. These include carbon monoxide CO, non-combusted hydrocarbons HC, nitrogen oxides NOx and particles. The nitrogen oxides are formed from the nitrogen of the combustion air during combustion or arise out of nitrogenous compounds in the fuel. Depending on the operating point of the engine, they consist of nitrogen monoxide NO in a proportion amounting to about 50 to 90 vol.-%. Furthermore, depending on the sulfur content of the fuel, the exhaust gas of internal combustion engines also contains sulfur dioxide.
With a view to keeping the air clean, certain upper limits for the emission of these harmful substances have been prescribed by the legislature. The limits are revised downwards from time to time, in accordance with the technical possibilities, in order to lessen the emissions of harmful substances. With a view to checking the conversion of harmful substances by exhaust gas emission control systems in a way approximating to practical reality, various test cycles have been established which simulate frequently occurring driving conditions.
The test cycle that is recognized in Europe in respect of a vehicle is designated as the MVEG-A cycle (Motor Vehicle Emission Group) and consists of an urban driving portion (ECE) and an extra-urban portion (EUDC). In this connection the vehicle to be examined covers the appropriate driving cycle on a roller dynamometer. The centrifugal-mass flywheels of the roller dynamometer constitute a collective load and simulate the weight of the vehicle. During the driving cycle the gaseous harmful substances which are emitted are measured and added up by means of standard analytical processes (HC: with a flame ionization detector (FID), CO: with infrared spectroscopy (IR), NOx: with a chemiluminescence detector (CLD)), so that the emissions of harmful substances of the entire cycle are available in the form of grams of harmful substance per kilometer travelled. These emissions can be directly compared with the limits specified by legislation.
With respect to their exhaust gas composition the present day internal combustion engines can be subdivided into two classes. On the one hand there are the conventional Otto (internal combustion) engines, the exhaust gas of which is composed substantially stoichiometrically, and on the other hand there are the diesel engines and lean burn Otto engines, the exhaust gas of which contains an excess of oxygen.
The exhaust gas of conventional Otto engines also exhibits, besides the stated harmful substances, an oxygen content of about 0.7 vol.-%. It is stoichiometrically composed; that is to say, oxidative and reductive components of the exhaust gas balance one another and can be converted simultaneously and almost completely with so-called three-way catalysts into the harmless components carbon dioxide, water and nitrogen. By way of catalytically active components, three-way catalysts contain on high surface area carrier oxides, in addition to platinum and/or palladium, also rhodium, which particularly favours the selective reduction of the nitrogen oxides to nitrogen by making use of carbon monoxide and non-combusted hydrocarbons as reducing agents. A prerequisite for simultaneous conversion of the three harmful substances by means of a three-way catalyst is the regulation of the air/fuel ratio supplied to the engine to the stoichiometric value. The latter amounts to 14.6 in the case of conventional fuels. Accordingly, 14.6 kilograms of air are needed for complete combustion of 1 kilogram of fuel.
In contrast, the exhaust gas of diesel engines and of lean burn Otto engines contains a high oxygen content amounting to about 6 to 20 vol.-%, since these engines are operated with lean air/fuel ratios. A major problem with these engines is constituted by the emission of the nitrogen oxides. On account of the high oxygen content of the exhaust gas it is not possible to convert the nitrogen oxides in accordance with the established three-way process using carbon monoxide and hydrocarbons as reducing agents. Instead, the oxidation of carbon monoxide and hydrocarbons in the oxygen-rich exhaust gas is preferred.
Frequently, therefore, the exhaust gases of these engines are only purified oxidatively with so-called oxidation catalysts; that is to say, carbon monoxide and hydrocarbons are oxidized on a catalyst by the oxygen in the exhaust gas to form water and carbon dioxide. Such a catalyst is described in DE 39 40 758 C1, for example. It is distinguished by a high catalytic activity in respect of the oxidation of carbon monoxide and hydrocarbons, whereas further oxidation of nitrogen monoxide to nitrogen dioxide and of sulfur dioxide to sulfur trioxide is largely suppressed. By this means, the formation of sulfates which could contaminate the catalysts that are used is also lessened. By way of catalytically active components the catalyst contains, on high surface area carrier oxides such as aluminum oxide, titanium oxide and silicon dioxide, platinum and/or palladium, which are modified in their catalytic activity as a result of additions of vanadium oxide in such a way that scarcely any further oxidation of nitrogen monoxide and sulfur dioxide is to be observed.
There have been various suggested solutions with a view to lessening the nitrogen oxides in the exhaust gas of these engines. In the case of so-called selective catalytic reduction (SCR) the nitrogen oxides in the exhaust gas are selectively reduced on an SCR catalyst by addition of reducing agents to the exhaust gas. The fuel itself can be used by way of reducing agent. Optimal results are achieved with ammonia as reducing agent, which, for example, can be generated on board the vehicle by hydrolysis of urea. Such a process is described in DE 42 03 807 A1. A suitable SCR catalyst is mentioned in EP 0 410 440 B1. It consists of an intimate mixture of the oxides of titanium, iron, vanadium, molybdenum, tungsten and various other additives.
Selective catalytic reduction can be employed economically only in the case of large diesel engines in trucks. Therefore the use of so-called nitrogen oxide storage catalysts has been proposed in recent years for the reduction of the nitrogen oxides in the exhaust gas of lean burn Otto engines. In this case the nitrogen oxides are oxidized to a higher state of oxidation on the storage catalyst by platinum group metals to form nitrogen dioxide and are stored in the form of nitrates by a basic storage material. After the storage capacity of the catalyst has been exhausted, it has to be regenerated. Regeneration is initiated by switching the operation of the engine from a lean air/fuel mixture to a rich, that is to say reducing, air/fuel mixture. In the reducing exhaust gas atmosphere the stored nitrates are decomposed, releasing nitrogen oxides which are converted into nitrogen under the reducing exhaust gas conditions on the platinum group metals. Accordingly, with this process, as also with the SCR process, reducing agents in the form of additional fuel are actively employed. To this end, suitable engine electronics are required which switch over periodically from lean running mode to rich running mode.
EP 0 669 157 A1 describes such a system. By way of storage material for the nitrogen oxides, use is made of basic materials such as alkali metal oxides, alkaline earth metal oxides and rare earth oxides. The storage catalyst additionally contains platinum and/or palladium on a high surface area carrier oxide.
With a view to improving the purification of exhaust gas and with a view to increasing the resistance to sulfur, various combinations of storage catalysts with other catalysts have become known. For instance, EP 0 716 876 A1 describes a catalyst which exhibits two porous carrier layers on a supporting body. The first carrier layer contains barium by way of storage material for the nitrogen oxides as well as palladium. The second carrier layer is situated on the first carrier layer and contains platinum which oxidizes nitrogen monoxide in the lean exhaust gas to nitrogen dioxide and thereby improves the storage of the nitrogen oxides by the first layer. In the stoichiometrically composed or rich exhaust gas the nitrogen oxides which are stored in the first layer are desorbed and reduced by palladium and platinum to elemental nitrogen. Palladium in the first layer is intended to protect the storage material against contamination by sulfur dioxide. By way of carrier materials for the first and second carrier layers, EP 0 716 876 A1 names aluminum oxide, silicon dioxide, aluminum silicate, titanium oxide and the like. Aluminum oxide is preferably employed as carrier material for both carrier layers.
With a view to improving the resistance to sulfur of the storage material, EP 0 664 147 A2 likewise describes a catalytic converter which in the direction of flow of the exhaust gas exhibits, firstly, a first catalyst consisting of a noble metal on a porous, acidic carrier material, a second catalyst consisting of a storage material for nitrogen oxides and a third catalyst consisting of a noble metal on a porous carrier material. The sulfur dioxide contained in the lean exhaust gas is neither adsorbed nor oxidized by the first catalyst and can therefore pass through the second catalyst without the formation of sulfates. In the rich or stoichiometrically composed exhaust gas the stored nitrogen oxides are released by the second catalyst and are reduced on the third catalyst to elemental nitrogen. By way of porous, acidic carrier materials for the first catalyst, SiO2, ZrO2, SiO2-Al2O3 and TiO2 are proposed.
WO 97/02886 describes a composition for the conversion of the nitrogen oxides in exhaust gases, which contains, closely adjacent to one another, a catalyst for the conversion of the nitrogen oxides and a material sorbing the nitrogen oxides. The catalyst for the conversion of the nitrogen oxides exhibits a highly dispersed platinum metal component on a first carrier material. The material sorbing the nitrogen oxides contains a basic metal oxide which is separated from the platinum group metal component. In a preferred embodiment, the catalyst for the conversion of the nitrogen oxides is applied in the form of a first coating on a supporting body. In this case the material sorbing the nitrogen oxides is applied onto the first coating in the form of a second coating. The two layers may also be interchanged. Metal oxides, metal hydroxides, metal carbonates and metal mixed oxides are described by way of storage compounds. The metals may be lithium, sodium, potassium, rubidium, caesium, magnesium, calcium, strontium or barium. The material sorbing the nitrogen oxides may contain, with a view to protection against contamination by sulfur, a component absorbing sulfur, preferably cerium oxide. This cerium oxide may be present in the form of particles alongside the particles of the storage material or may be dispersed in the nitrogen oxide storage compound.
With a view to removing the nitrogen oxides from the exhaust gases with the catalyst arrangement according to WO 97/02886, the composition of the exhaust gas is switched back and forth periodically between lean and stoichiometric, orxe2x80x94to be more exactxe2x80x94rich, by appropriate control of the air/fuel ratio.
WO 97/43031 proposes a process for removing the nitrogen oxides from the exhaust gas of, in particular, diesel engines. In this case the exhaust gas is firstly conveyed over a nitrogen oxide storage components and subsequently over a nitrogen oxide reduction catalyst. The nitrogen oxide storage component contains a combination of an oxidation catalyst and a storage material. Upstream of the nitrogen-oxide storage component, hydrocarbons are periodically injected through a nozzle into the exhaust gas in order to desorb the sorbed nitrogen oxides thermally. Without further measures this results in a mean conversion of nitrogen oxide amounting to zero. In addition, therefore, upstream of the reduction catalyst hydrocarbons must again be injected through a nozzle into the exhaust gas in order to obtain a net conversion of the nitrogen oxides.
According to WO 97/43031, the hydrocarbons which are injected through a nozzle upstream of the nitrogen oxide storage components are combusted on the oxidation catalyst of the nitrogen oxide storage component. Their quantity is such that the composition of the exhaust gas remains lean but the heat that is released in the course of combustion on the nitrogen oxide storage component is sufficient to desorb the stored nitrogen oxides thermally.
Accordingly, the known processes for operating storage catalysts all require a periodic raising of the hydrocarbon content of the exhaust gas, in order either to decompose the stored nitrogen oxides under rich or stoichiometric exhaust gas conditions or to desorb them thermally by increasing the temperature on the storage catalyst. Raising of the hydrocarbon content of the exhaust gas is effected in this case either by diminishing the air/fuel ratio supplied to the engine or by injecting fuel into the exhaust system through a nozzle downstream of the engine.
To the extent that they require an enrichment of the air/fuel mixture, these active processes for lessening the nitrogen oxide emissions of lean burn engines are unsuitable for use in diesel engines, since the latter operate flawlessly only with a constantly lean air/fuel mixture. By virtue of the enrichment of the air/fuel mixture or by virtue of the injection of fuel through a nozzle into the exhaust gas, the active processes result in an increased consumption of fuel.
The increased consumption of fuel should be avoidable, particularly in the case of diesel engines, since these engines exhibit relatively low nitrogen oxide emissions. This is because the non-combusted hydrocarbons, carbon monoxide and hydrogen which are still contained in the exhaust gas of diesel engines represent, on average, a sufficient quantity of reducing agents in order largely to reduce the low nitrogen oxide emissions.
In the case of diesel engines, therefore, attempts are made to improve the diminution of the nitrogen oxides also without enriching the exhaust gas by using only the reductive constituents that are contained in the exhaust gas in any case. A suitable catalyst for this is described in DE 196 14 540. On account of the low selectivity of the reduction of nitrogen oxide and by reason of the competing direct oxidation of the reductive constituents by the high oxygen content of the exhaust gas, the achievable degrees of conversion in this case are low. Only under optimal conditions, that is to say in the case of a uniformly high proportion of hydrocarbons in the exhaust gas, are rates of conversion of 60% achieved. Over the so-called MVEG-A test cycle, however, such a catalyst only permits a conversion of about 14%.
An object of the present invention is therefore to find a catalyst for cleaning up the continuously lean exhaust gas of diesel engines, which enables better utilization of the reductive components contained in the exhaust gas for the reduction of the nitrogen oxides and in this way results in a higher reduction of nitrogen oxide, averaged over the driving cycles occurring in practice, than known reduction catalysts.
The above and other objects of the invention can be achieved by means of a catalyst for cleaning up the exhaust gases of a diesel engine, which contains two functional layers superimposed on an inert supporting body, whereby the first layer, which is situated directly on the supporting body, has a nitrogen oxide storage function and the second layer, which is in direct contact with the exhaust gas, has a catalytic function. A feature of the invention is that the second functional layer of the catalyst additionally has a hydrocarbon-storage function and its catalytic function is provided by catalytically active noble metals of the platinum group which are deposited in highly dispersed form on finely divided, acidic carrier materials.
Within the scope of this invention, the term xe2x80x9cfunctional layersxe2x80x9d is to be understood to mean dispersion coatings on an inert supporting body which are able to change the chemical composition of an exhaust gas streaming past. The changes may consist in certain exhaust gas components being removed, at least temporarily, from the exhaust gas as a result of adsorption on constituents of the functional layers. Thus storage materials for nitrogen oxides are known that store the nitrogen oxides from the exhaust gas in the form of nitrates. Similarly, zeolites are known for the storage of hydrocarbons contained in the exhaust gas.
A further influence on the chemical composition of an exhaust gas streaming past resides in that certain harmful substances with other components of the exhaust gas are converted into harmless products upon contact with the constituents of the functional layer. This catalytic function is preferably provided by the noble metals of the platinum group, in particular by platinum, palladium, rhodium and iridium, which, with a view to full display of their catalytic activity, are deposited in highly dispersed form on finely divided support materials. Their catalytic activity can be influenced by addition of so-called promoters. In this connection it is a question, for the most part, of compounds of base metals.
With a view to characterizing the type of the catalytic activity, mention is frequently made of oxidation catalysts, reduction catalysts or three-way catalysts. However, these three catalytic functions cannot be clearly separated from one another. The function that actually expresses itself distinctly for a given composition of the functional layer also depends on the composition of the exhaust gas. A three-way catalyst can only convert carbon monoxide, hydrocarbons and nitrogen oxides simultaneously when the exhaust gas is composed stoichiometrically. In order that a so-called reduction catalyst can display its reducing activity, the exhaust gas has to contain enough reducing components; that is to say, carbon monoxide, hydrocarbons and hydrogen. The reducing components are then oxidized in the process.
Within the scope of this invention, the term xe2x80x9ca finely divided materialxe2x80x9d is to be understood to mean a pulverulent material which is introduced as such into the catalyst. In the English language patent literature the term xe2x80x9cbulk materialxe2x80x9d or xe2x80x9cparticulate materialxe2x80x9d is used for this. These materials are frequently employed as support materials for catalytically active components or other highly dispersed constituents of the catalyst. For this purpose the support materials have to exhibit a high specific surface area (also BET surface areas measured in accordance with DIN 66132, for example) for the acceptance of these components. Within the scope of this invention, the finely divided materials are designated as high surface area materials if their specific surface area amounts to more than 10 m2/g.
The highly dispersed materials are to be distinguished from the finely divided materials. Highly dispersed materials may, for example, be deposited on finely divided, high surface area support materials by impregnation. To this end, the support materials are impregnated with, as a rule, water-soluble precursor compounds of the highly dispersed materials. By means of an appropriate temperature treatment the precursor compounds are then transformed into the highly dispersed materials. The particle size of these highly dispersed materials ranges from about 5 to 50 nm.
Within the scope of this invention, the term xe2x80x9cstorage componentsxe2x80x9d is used to designate the elements of the alkali metals and alkaline earth metals. In this connection it is preferably a question of potassium, rubidium, caesium, magnesium, calcium, strontium and barium. They form strongly basic oxides which are able to bind the nitrogen oxides. The oxides of the storage components are therefore also designated as storage compounds or active storage compounds. But the term xe2x80x9cstorage compoundxe2x80x9d are used herein is also to be understood to mean the products of reaction of the oxides with air or with the exhaust gas components to form carbonates and hydroxides, which are likewise capable of storing nitrogen oxides. The storage capability of the storage compounds is generally the greater, the stronger their basicity.
The storage materials are to be distinguished from the storage compounds. In the case of the storage materials it is a question of support-based storage compounds; that is to say, of storage compounds which are deposited in highly dispersed form on suitable support materials. But, within the scope of this invention, storage compounds which are present in finely divided form are also designated as storage materials.